1. Field of the Invention
This invention relates generally to disc cylinder locks, and is concerned in particular with an improvement designed to render it extremely difficult and in most cases virtually impossible to counterfeit the keys used to operate such locks.
2. Description of the Prior Art
Disc cylinder locks are well known and have been in widespread use for over seventy years. Examples of recent developments in locks of this type are described in U.S. Pat. Nos. 4,637,234 and 4,742,703.
The conventional disc cylinder lock typically comprises a tubular lock barrel containing a rotatable cylinder. The barrel has a longitudinally extending internal groove and the cylinder has a parallel slot containing a radially shiftable locking bar. The cylinder further contains a series of axially stacked independently rotatable operating discs having mutually aligned specially configured central apertures defining a keyway. The peripheral edges of the discs are interrupted by notches and the lock is operated by a key insertable in the keyway. The key has a series of longitudinally spaced angularly offset external flats, each being arranged to mate with an internal flat bordering the central aperture of a respective one of the operating discs. When the discs are "scrambled", at least some of their peripheral notches are out of radial alignment with the cylinder slot, with the result that the locking bar is urged outwardly and caused to protrude from the cylinder slot into the internal barrel groove, thereby preventing the cylinder from turning in the barrel. Rotation of the key brings the respective external and internal mating flats of the key and discs into coactive engagement, causing the discs to rotate until their respective peripheral notches are all in radial alignment with the cylinder slot, at which point the locking bar is permitted to shift radially inwardly from the barrel groove. This frees the cylinder to turn within the barrel.
It will thus be seen that the operating discs act in a similar manner to tumblers in a safe, making the lock extremely difficult to pick. Moreover, by varying the number of operating discs and/or the orientation of the mating sets of internal disc flats and external key flats, a myriad of combinations can be developed. This latter feature is important in applications where it is desirable to provide each lock and key set with its own unique combination.
However, there are other applications, such as for example in security systems for public utility meters, where tens of thousands of identical locks must be provided with the same combination so that all may be operated by identical keys distributed to utility personnel. Here, a major concern centers on preventing genuine keys from being counterfeited. Once this happens, the integrity of the entire system is compromised, thus making it necessary to replace all of the locks.
The problem with conventional disc cylinder locks is that the arrangement of angularly offset external flats on their operating keys is relatively easy to copy. This can be done most readily by taking measurements directly from a genuine key, but it can also be accomplished by dismantling a lock and taking measurements from the internal flats on the operating discs. Because of this problem, public utilities have been reluctant to employ disc cylinder locks in their security systems.